Modern signal transmission cables typically are shielded by a thin conductive foil and include a drain wire in contact therewith, running the length of the cable, that is used to terminate the foil shield. Such a transmission cable is shown in FIG. 1 at 10. The cable 10 includes a pair of insulated signal conductors 12 and 14 and a non-insulated drain wire 16 all of which are arranged side by side as shown. A layer of conductive shielding material 18 is wrapped around the three conductor assembly so that it is in electrical contact with the non-insulated drain wire. This shielding prevents emissions from the cable as well as provides isolation from nearby or stray signals, and the planar structure of the cable provides advantages in routing and other cable management tasks for certain applications. When this cable is used in differential logic applications with relatively fast rise times and high bit rates, the propagation delay of the signal along the two signal conductors 12 and 14 becomes important. The air gaps 20, as seen in FIG. 1, result in asymmetrical capacitive coupling between the shield and the two signal conductors. The dielectric constant is different for each one because the air gaps affect the signal on the conductor 12 more than the signal on the conductor 14, thereby causing different propagation delays for the two signals. In fast switching circuitry, high speed clocklines, and long-run cable configurations this difference can cause the output signal to either not reach the threshold value or, if it does, the signal pulse may be so narrow that it will lack sufficient energy to register as a data bit thereby causing a parity error. A solution to this problem is to arrange the drain wire in the space 22, against the outer insulation of the two signal conductors. However, this adds a bulge in the otherwise flat surface of the cable thereby adversely affecting installation in many applications. Additionally, such an arrangement makes it difficult to terminate the drain wire by automated equipment.
What is needed is a transmission cable having signal conductors with substantially similar propagation delays while maintaining the desired flat profile afforded by arranging the drain wire on the same center line as the two signal conductors.